The present invention is directed to a method and apparatus for producing a multiple diode semiconductor laser array having accurate interlaser spacings on the order of less than 100 .mu.m. Such an array may be incorporated in numerous devices including flying spot scanners (commonly referred to as raster output scanners (ROSs)). A flying spot scanner typically has a reflective multifaceted polygon mirror that is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a linear or fast-scanning direction. Printers employing multiple intensity-modulated beams are referred to as multibeam or multispot printers. The multispot lasers are considered to be an enabling technology for high speed printers operating at resolutions of about 600 spots per inch (spi) or greater. The present invention includes multiple laser dies bonded to a heatsink in a linear array to provide the multibeam output capability for a ROS. Furthermore, the relative spacing between the laser emitting regions of the laser dies is accurately controlled by alignment means between the heatsink and the laser dies.
Heretofore, the desirability of a multiple beam semiconductor laser arrays has been recognized. Designs intended to achieve close spacing of the emitted laser beams are known, of which the following disclosures which may be relevant:
U.S. Pat. No. 4,393,387
Patentee: Kitamura
Issued: Jul. 12, 1983
U.S. Pat. No. 4,404,571
Patentee: Kitamura
Issued: Sep. 13, 1983
U.S. Pat. No. 4,420,761
Patentee: Kitamura
Issued: Dec. 13, 1983
U.S. Pat. No. 4,474,422
Patentee: Kitamura
Issued: Oct. 2, 1984
U.S. Pat. No. 4,690,391
Patentee: Stoffel et al.
Issued: Sep. 1, 1987
U.S. Pat. No. 4,712,018
Patentee: Stoffel et al.
Issued: Dec. 8, 1987
U.S. Pat. No. 4,914,667
Patentee: Blonder et al.
Issued: Apr. 3, 1990
U.S. Pat. No. 4,999,077
Patentee: Drake et al.
Issued: Mar. 12, 1991
The relevant portions of the foregoing patents, hereby incorporated by reference for their teachings, may be briefly summarized as follows:
U.S. Pat. No. 4,393,387 teaches a beam recording apparatus including a semiconductor array laser light source having a plurality of light beam emitting points, a condensing optical system, an image rotator, and a rotatable polygon mirror for deflecting the light beams to the surface of a photosensitive drum. High density recording is enabled by controlling the angle of incidence, and therefore the interbeam spacing, of the outermost beams at the surface of the photosensitive drum.
U.S. Pat. No. 4,404,571 describes a multibeam recording apparatus comprising a scanner for scanning a recording medium with a plurality of light beams and a beam detector. The scanner employs a laser array light source having a plural number of semiconductor lasers arranged in a row. The beam detector utilizes a screen plate with a detection aperture which is smaller than the inter-beam spacing to individually detect each of the plural beams.
U.S. Pat. No. 4,420,761 discloses a recording apparatus having plural dots arranged inclined to the scanning direction in order to increase the dot or scanning density. The phase difference caused by such inclination of a semiconductor laser array is compensated for using delay circuitry to generate a delay in the driving signals for the plural laser beams in accordance with the offset caused by the inclination.
U.S. Pat. No. 4,474,422 describes an optical scanning apparatus having a light source consisting of an array of aligned light sources. The beams from the light sources are collimated and deflected to sweep across a single photoreceptor. The beams are also displaced from each other in the cross-scan direction so that multiple lines can be scanned simultaneously across the photoreceptor. An object of U.S. Pat. No. 4,474,422 is to reduce variations in pitch by closely spacing individual lasers within the laser array in a compact structure.
U.S. Pat. No. 4,690,391 and U.S. Pat. No. 4,712,018 teach a method and apparatus, respectively, for the fabrication of long, full width arrays of reading or writing elements from a plurality of smaller arrays. An alignment tool, having array aligning formations thereon, is utilized to align the smaller arrays by having the aligning formations contact an array aligning formation present on the smaller array. The illustrated embodiment utilizes pins present on the surface of the alignment tool to mate with V-grooves on the smaller array.
U.S. Pat. No. 4,914,667 discloses a laser light source for use with optical communications. More specifically, the laser is a Bragg-reflector type laser which is aligned, via a V-groove etched in a supporting substrate, with an optical fiber. Vertical and lateral alignment is achieved by the selectively etched rail and groove features, while the remaining degree of freedom is used to abut the laser active portion against the Bragg-reflector portion.
U.S. Pat. No. 4,999,077 teaches a method of fabricating a coplanar array from a plurality of short scanning subunits for reading and writing images. Fabrication is accomplished using an alignment fixture having a patterned thick film layer on a surface thereof. The subunits, having correspondingly keyed strips on a surface thereof so that when placed in contact with the alignment fixture the subunits are maintained in an abutting relationship. Subsequently, the subunits are adhesively attached to a structural member which is placed in contact with an opposite surface, thereby forming a coplanar array.
An object of the present invention, therefore, is to enable the assembly of an array of individual semiconductor lasers, each of which may have different characteristics. For example, each of the lasers may have a different wavelength, polarization or other characteristic which makes it difficult to fabricate such an array monolithically.
In accordance with the present invention, there is provided a multiple diode laser array comprising: a heatsink, a plurality of laser diodes, each including a wafer having a series of epitaxially grown layers deposited upon a surface thereof to form an emitter, and a ridge-shaped waveguide defining the position of the emitter, and alignment means, disposed at preset intervals along a planar surface of said heatsink and adapted to associate with the waveguides of said plurality of laser diodes, for controlling the spacing of said laser diodes with respect to one another to assure accurate spacing of the emitters therein.
In accordance with another aspect of the present invention, there is provided a single diode laser including a wafer portion having a series of epitaxially grown layers deposited upon a surface thereof to form a p-n junction emitter and a waveguide present on the surface of the wafer to define the position of the emitter, the single diode laser being adapted to be assembled adjacent to like single diode lasers to provide a multiple diode array. The array further comprises a heatsink alignment means, disposed at along a planar surface of said heatsink, for controlling the location of said laser diodes, said alignment means being adapted to remain in an abutting relationship with the waveguides of the laser diodes so that a predetermined spacing between the emitters of adjacent laser diodes is accurately maintained, and means for permanently affixing the laser diodes to the heatsink.
In accordance with yet another aspect of the present invention, there is provided a method of fabricating an array of multiple diode lasers, each laser having a raised waveguide on a surface thereof which defines the location of light emitting regions thereon. The method comprising the steps of:
1) forming a plurality of parallel alignment structures on a surface of a heatsink, said alignment structures being spaced apart from one another while extending in a substantially parallel direction, thereby forming elongated recesses therebetween;
2) placing the lasers on said heatsink so that the lasers are maintained in a predetermined spaced-apart arrangement by an abutting contact region between a surface of the raised waveguide and a surface of the alignment structure; and
3) permanently affixing said laser diodes to said heatsink.